There are some issues when you try to delete a domain from forefront.
Follow these steps:
a. Call Forefront support and tell them to delete the domain for you.
b. Run these commands in your powershell session :
1) Launch Windows PowerShell (Run as Administrator)
****If this is your first time connecting to O365 from your computer, please run the following command first before proceeding to step 2:
The execution policy helps protect you from scripts that you do not trust. Enter “Y” and proceed to step 2 below.
2) Enter $LiveCred = Get-Credential command. Enter the credentials of an administrator account in the O365 Company to login.
3) Issue the following command to create a connection.
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic –AllowRedirection
4) Run the Set-ExecutionPolicy RemoteSigned command. This command is required for FIRST TIME connecting to O365.
5) Run the Import-PSSession $Session command
6) First, runs the Set-AcceptedDomain DOMAINNAME.COM –OutboundOnly $true command
***NOTE: This command will convert the domain in question into a duplicate domain name.
7) Then, runs the Set-AcceptedDomain DOMAINNAME.COM –OutboundOnly $false command
***NOTE: This command will convert the duplicate domain name into a normal domain name.